PROJECT SUMMARY Skeletal muscles can regenerate throughout lifetime by using resident muscle stem cells. Muscle stem cells are normally quiescent. Upon injury stimuli, they can proliferate to make new muscles, i.e. regeneration. During regeneration, muscle stem cells also renew themselves and return to quiescence so they can foster future rounds of regeneration. During the aging process and under certain experimental conditions, muscle stem cells can also break quiescence and proliferate without injury stimuli. However, under these conditions muscle stem cells do not return to quiescence and become lost. Loss of muscle stem cells negatively impacts muscle regeneration. What drives muscle stem cells to proliferate without injury pertains to a cellular state of `proliferation competence' within quiescent muscle stem cells. We are interested in whether the molecular machinery for this `proliferation competence' during quiescence shares similarity with or differs from injury stimuli-induced proliferation program. We have previously shown that the cyclic-AMP-responsive-element binding protein (CREB) family (CREB, CREM, and ATF1) of transcriptional activators regulates skeletal myogenic progenitor fate in the mouse embryo. To test whether the CREB family plays a role in adult muscle regeneration, we inhibited the CREB family in muscle stem cells. We found that CREB family is need to activate a set of genes in quiescent muscle stem cells, but not needed to maintain their quiescence. However, muscle stem cells with CREB family inhibited cannot proliferate or renew following injury stimuli. We identified a protein called Mpp7 as a candidate mediator acting downstream of CREB family to drive muscle stem cell proliferation and renewal. We propose that this protein, together with one of its associated protein complexes, constitute common core machinery for proliferation competence during quiescence and injury-induced proliferation. Our proposed research is aimed to determine whether and how Mpp7 instructs muscle stem cell function: 1) We will determine genetically whether Mpp7 is indeed critical for muscle stem cell proliferation and renewal; 2) We will investigate whether a Mpp7-associated protein complex is responsible for proliferation and renewal of muscle stem cells using siRNA screens and genetic studies; 3) We will further determine the biochemical and molecular mechanisms underlying proliferation and renewal of muscle stem cells driven by the Mpp7-associated protein complex, and explore its potential involvement in mechano-sensing.